The present invention relates to a master cylinder for supplying a brake fluid to a brake apparatus of a vehicle.
There is known a master cylinder for supplying a brake fluid to a brake apparatus of a vehicle, which comprises a cylinder body into which a plunger-type piston is directly inserted. In this master cylinder, to allow communication between a circumferential groove in which a piston seal is fittingly disposed and a brake fluid discharge passage which is formed on a side of the bottom of the cylinder body relative to the circumferential groove, an annular large-diameter groove is formed in the cylinder body on a side of the discharge passage, and a communication groove in a straight form connecting the large-diameter groove and the circumferential groove is formed in an upper portion of the cylinder body. When releasing air from the cylinder body, the piston seal is opened under differential pressure, and the brake fluid is flowed toward the discharge passage through a gap between the piston seal and the bottom of the circumferential groove, and the communication groove and the large-diameter groove (see, for example, Japanese Patent Public Disclosure No. 2004-231093). In this master cylinder, the communication groove is located inward of the bottom of the circumferential groove, relative to a radial direction of the cylinder body. Therefore, when releasing air by applying suction, due to the action of a negative pressure in the cylinder body, an end portion near the outer circumferential surface of the piston seal adheres to a side wall surface rising from the bottom of the circumferential groove, thereby preventing the brake fluid from flowing into the communication groove through the gap between the outer circumferential surface of the piston seal and the bottom of the circumferential groove. Therefore, the supply of the brake fluid is restricted and an air releasing operation must be repeated several times.
Therefore, it has been proposed to use a master cylinder in which a communication groove in an arcuate form for connecting the discharge passage and the circumferential groove in an upper portion of the cylinder body is formed outwardly beyond the bottom of the circumferential groove, relative to the radial direction of the cylinder. In this master cylinder, when releasing air, a brake fluid can be satisfactorily supplied through a gap between the outer circumferential surface of the piston seal and the bottom of the circumferential groove, and the communication groove, so that the number of repetitions of an air releasing operation for complete release of air can be minimized (see, for example, Japanese Patent Public Disclosure No. 2004-299568).
In the above-mentioned master cylinder, the center of arc of the arcuate communication groove is offset from the center axis of a slidable inner-diameter portion of the cylinder body. In addition, the communication groove is required to have a substantial length to be formed. Therefore, a substantial time is required for machining, which increases cost of manufacture.